Anatomy of a firestorm

Victoria's climate has long set the scene for annual danger. William Kininmonth

IN THE evolving history of Victoria there will be some days never forgotten because of the magnitude of lost lives and devastation from uncontrollable fire.

The Healesville to Marysville road after Black Friday, 1939.

Black Thursday was February 6, 1851, when one-quarter of the emerging colony of Victoria burned. February 14, 1926, does not have a special name but 60 lives were lost in widespread fires. Seventy-one lives were lost on Black Friday, January 13, 1939, and vast areas of forest were destroyed in fires that continued for another eight days. On Ash Wednesday, February 16, 1983, fires erupted from the Adelaide Hills to east of Melbourne and 47 Victorians died.

Black Saturday, February 7, 2009, is now added to this dreadful roll call.

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These were not the only days of fire devastation, just the worst. The climate of Victoria with its winter rains and summer heat sets the scene for annual danger. Grasses and forest vegetation grow prolifically in the warm moist conditions of spring and dry out over summer into a tinderbox, just waiting for an ignition. And the ignition generally comes, either naturally as in lightning strikes, through accidents or from arson. But not all fires reach the intensity and spread with the ferocity of our worst.

Two characteristics mark the historic fire days. The first is a long period of significantly below-average rainfall; the second is special meteorological conditions on the day, with a combination of high temperatures, low humidity and strong winds.

Bureau of Meteorology published records show that since 1900 the average annual rainfall for Victoria has varied from 356 millimetres (in 1967) to 919 millimetres (in 1973). The same records also show that the rainfall for the year before each major fire day ranked among the lowest: 1925 ranked 16th lowest at 511 millimetres; 1938 ranked fifth at 412 millimetres; 1982 ranked second at 362 millimetres; and 2008 ranked 15th at 504 millimetres.

Below-average rainfall during the preceding winter and spring does not allow the soil moisture to become fully replenished. As a consequence, the grasses cure early and the forests become extraordinarily dry during the following summer.

Rainfall is highly variable from year to year and more frequent low or high annual rainfall years have been characteristic of given decades. The decade from the mid-1930s to the mid-1940s was particularly dry and this pattern has been repeated over the recent decade.

In contrast, some very high rainfalls in the 1950s and the 1970s made those particularly wet decades. Broadly speaking, the first half of the 19th century was drier than the second and rainfall has now returned to the early 20th century pattern.

There is no simple explanation for the varying rainfall, although the changing patterns of surface temperature over the warm Indian and Pacific oceans are known to be important but complex influences. To a large extent it is the changing tropical patterns of ocean surface temperature that regulate the transfer of heat and moisture to the atmosphere, and also the location and intensity of tropical convection.

The weather systems responsible for Victoria's rainfall rely on a supply of moisture from the tropics. When the focus of tropical convection shifts from the western Pacific to the central or eastern Pacific, such as during an El Nino event, the weather systems affecting Victoria can be starved of their essential moisture. Conversely, La Nina events, with their focus of tropical convection more to the north and west of Australia, are a more favourable source of moisture.

In addition to an appropriate source of tropical moisture there is need for a favourable air current to bring the tropical moisture across Victoria to feed into passing rain systems. Ideally, the moist air ascends and converges into the clouds of the rain system and widespread rainfall results. Ideal conditions are the exception rather than the rule.

To a large extent it is a matter of chance whether a potential rain system moving over Victoria accesses a favourable moisture supply. If the tropical moisture source and the middle and high-altitude westerly airflow across southern Australia, known as Rossby Waves, provide a good link then good rains ensue. However, if either the tropical moisture source or the persisting location of the Rossby Waves is unfavourable then seasonal rainfall is low.

Even when there has been low rainfall in the preceding winter and spring it takes a special conjunction of meteorological processes to bring the extremely high temperatures, dry air and strong winds that are characteristic of the mega-fire events. A detailed investigation will be made of the meteorology leading up to and during the recent fires as part of the Royal Commission, but there are general characteristics that are already apparent from the available information. What will need to be evaluated is why these characteristics achieved such intensity.

The analysis of the meteorology of the 2009 fires will be assisted by observing systems and data that were not available during earlier disastrous bushfires. The Bureau of Meteorology's sophisticated satellite surveillance, its network of radar stations and the network of automated meteorological observing stations not only supported the excellent monitoring and early warning services but, with the accumulated climate data, will give new insights into how the extremely hazardous conditions developed.

Daily temperature and humidity over Victoria during summer follows a bi-modal pattern. When the winds are from the southern sector the temperatures are typically mild and humidity is relatively high, having origins from the oceans to the south. When the winds come from the north from the continental interior temperatures are hotter.

Wind direction alone is not sufficient to explain the extremely high temperature and low humidity experienced on Black Saturday, Ash Wednesday or Black Friday. The air spreading across the Victorian countryside had its origins in the middle atmosphere where it is naturally dry. As it descended to the surface the air warmed by compression in the denser air and brought the strong winds of the middle atmosphere with it. To a large extent, the surface temperatures that were reached are a reflection of the altitude from which the air originated.

In the weeks leading up to the major fire event the middle atmosphere airflow across southern Australia had been from the north-west, extending from the Indian Ocean to the Tasman Sea. This in itself was not unusual. It was the heavy monsoonal rains and cyclonic activity that had deluged the north of Australia that gave special character to the situation.

Widespread buoyantly ascending air associated with the clouds and rain of the tropical rain required compensating mass subsidence elsewhere. This was occurring in the north-westerly airflow, drying it out and contributing to the long dry spell over southern Australia through January.

A cooler air mass formed a southern boundary to the dry subsiding air. The boundary extended from north of Perth, crossing the south coast to the east of Esperance and extending to Tasmania. A long high-level cloud line marked the air mass boundary for much of the time in the days leading to the fire event. Radar images suggested light rain at times to the east of Esperance in the cooler air and on the morning of the fires there was rain over and offshore from western Tasmania. On the eastern side of the boundary the air was hot and dry.

On the morning of Black Saturday the air mass boundary acted as a barrier to the air spreading from the tropics and subsiding air was channelled over south-eastern Australia. The dry air from the middle atmosphere was forced downwards in the north-westerly airflow and heated by compression as the density increased near the surface. As the sun heated the ground, the stability of the airflow decreased and the air was able to mix through a great depth bringing wind strength from the middle atmosphere to the surface.

It was the combination of the special dynamics of the airflow and the heating of the surface that brought about the high temperatures, the low humidity and the strong north-westerly winds. On Black Saturday 2009 new temperature records were set over a wide area, including 46.4 degrees in Melbourne with a relative humidity of less than 6 per cent. By comparison, fire-day temperature in 1926 was 39.9 degrees; Black Friday 1939 was 45.6 degrees (the previous record); and Ash Wednesday 1983 was 43 degrees.

Compounding the impact of the hot dry air was the strong winds as air from the middle atmosphere was brought to the surface. The effect of the winds was clearly observable on the Bureau of Meteorology radar as fires developed north of Melbourne. The initially small plumes rapidly intensified and spread south-east to cross the Gippsland coast. Once the initial fires became established in the hot dry airflow the burning debris and embers were carried ahead to spot and start secondary fires, which themselves developed to become sources of new embers.

Horrendous as the fire conditions were during the day, they were compounded late in the afternoon when the air mass boundary moved inland as a cool change. All of a sudden the now extensive north-eastern boundary of the fires became a leading edge as the strong south-westerly winds of the cool change advanced. The air ahead of the change was still dry, temperatures were high, and the firefront was fanned by strong winds. Even after the cool change had passed there were remaining fires fanned by strong winds that posed a continuing danger.

A point to be emphasised is that the meteorology of extreme fire days develops and persists over a period of many days. Black Friday of 1939 was part of a heatwave over south-eastern Australia that sequentially, over a week, brought record temperatures to Adelaide, Melbourne and Sydney.

A week before Black Saturday 2009, Melbourne had a series of three days with temperatures in the low 40s with intervening days about 30 degrees as temperatures were tempered by sea breezes.

The excellent early warnings, on-day predictions and real-time advice given by the Bureau of Meteorology on Black Saturday point to the importance of maintaining the modern infrastructure that supports these essential services. Megafires fortunately are a relatively rare event but each summer has the potential for another catastrophe.

It is fashionable to promote climate change as being a contributor to changing fire frequency and intensity. The pattern of rainfall over the past century does not point to a trend of reduction in rainfall. Nor has any link been offered between global temperature trends and the meteorology of Victorian heatwaves. Extreme bushfire events are rare events and must be analysed according to the statistics relating to rare events; the breaking of a previous temperature record established 70 years earlier does not establish an underlying trend.

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The firestorm conditions of Black Saturday 2009 resulted from the earlier conditioning of vegetation and the unusually intense meteorology of the heatwave that established hot dry airflow and accompanying strong winds over a wide area. While it is possible to broadly identify and monitor the characteristics there is much to be learned about the large-scale controls over seasonal rainfall and the unique meteorology of heatwaves. Ongoing research, early adoption of new technologies and the maintenance of robust meteorological infrastructure are essential strategies forming part of our community defence against future megafires.

William Kininmonth is a former head of Australia's national Climate Centre. He has been a consultant to the World Meteorological Organisation on climate issues and is author of Climate Change: A Natural Hazard (Multi-Science Publishing Co., UK 2004).